As device geometry becomes smaller and smaller, interconnects used in devices become thinner and thinner. To obtain low resistance interconnects, metals such as aluminum and copper are used extensively because both metals have low resistivity values and low cost. These materials are usually deposited by sputtering processes. However, other methods, such as evaporation and CVD, are also used in particular applications.
One of the problems in reducing the size of devices, and especially the thickness of the conductors, is due to the fact that the resistivity of thin films increases with the decreasing thickness if the thickness approaches values below 20 nm. This is due to the fact that surface scattering of electrons begins to make a noticeable contribution to the resistivity. This contribution is in first approximation inversely proportional to the thickness of the film and is also a function of how many electrons are scattered inelastically at the surface. Another contribution to the resistance is due to geometric effects. Below 5 nm, metal films are usually not continuous, because the metal atoms tend to form islands. Conduction requires that electrons jump from one island to another, leading to high resistence values.
Chopra (Thin Film Phenomena, McGraw Hill, New York, N.Y. 1969, p357) published resistivity values of gold films as a function of thickness t. The resistivity was nearly constant for t>40 nm, but increased rapidly with decreasing t for t values below 25 nm. The Handbook of Thin Film Technology (L. I. Maissel and R. Gland, Edt., McGraw Hill, New York, N.Y. 1983, p 13-12) shows that the resistance of annealed gold films is nearly constant between 20 and 85 nm. There is an increase in the resistivity with decreasing thickness below t=20 nm. The lowest data points are given for t=4 nm.
I. M. Rycroft and B. L. Evans (Thin Solid Films 290-1, 1996, pp 283-288) measured the resistivity of Pt, Ni, Fe, Cu, and Ag samples. The iron film deposited on glass showed a continuous region for t>6 nm, a semi-continuos region for 2 nm<t<6 nm, and a discontinuous region for lower thickness values. The resistance/square was about 108 Ohm/square for the 1 nm thick film, at t=2 nm the resistance/square was close to 106 Ohm/square, and at t=6 nm the resistance/square was about 103 Ohm/square. The transition from the semi-continuous region to the continuous region occurred for six of the samples between 2.3 nm and 5.1 nm, while two others showed values of 7.5 nm and 22 nm.
R. Schadt, S. Heun, T. Heidenblut and M. Henzler (Phys. Rev. B45, pp 11430, 1992) studied thin epitaxial Ag films on Si(111) surfaces at very low temperatures. The Si-substrate was subjected to a complicated cleaning and annealing process to produce atomically clean and smooth surfaces. Films thicker than 2 monolayers behaved metallically. Resistivities were measured at 20 K. The resistivity of the 1-monolayer thick film (t=0.2 nm) was about 0.01 Ohm·m, while for the 5-monolayer thick film (t=1 nm) the resistivity was about 10-4 Ohm·m.
Very low resistance values for thin metal films deposited in a high vacuum on MgF2 or polymers were found by Rasch and Cowden (U.S. Pat. No. 3,801,325). In one case a 6 nm thick Mn-film was deposited on a 5 nm thick MgF2-film, which was then covered by another layer of MgF2. The resistance/square was <104 Ohm. Their thinnest Al-films (2.2 nm thick) deposited on polyethylene terephthalate (PET) had a MgF2 overlayer. The film had a resistance of 200 Ohm/square. The patent disclosure deals only with the use of these films in photographic articles; the possible use in electronic devices or in any other application was not mentioned.
K. Schroder and Le Zhang (Phys. Stat. Sol. (b) 183, pp K5, 1994) determined that chromium films deposited on a germanium substrate showed very low resistivity values. The resistivity of the 0.3 nm thick film was 5×10−6 Ohm·m, and the 1 nm thick film showed a resistivity of 10−6 Ohm·m. The results obtained for chromium films were regarded as unusual for metal films. Chromium films show unexpected magnetic and thermopower properties, which suggested that the films could be superconducting at the surface at room temperature (K. Schroder and S. Najak, Phys. Stat. Sol. (b) 172, pp. 679, 1992. K. Schroder, Le Zhang, and W.-T Ger, Phys. Stat. Sol. (b) 181, pp. 421, 1994). This was predicted in the exciton theory proposed by Bardeen, who showed theoretically that the transition temperature from superconductivity to normal behavior can be very high at the interface of a superconductor and a semiconductor.
In U.S. Pat. No. 6,406,997 (Schroder), it was disclosed that chromium overlayers on conductors should reduce failure by electromigration, because the interface resistance between the chromium overlayer and a conductor can be lower than the surface resistance of the conductor without an overlayer. It was argued that a low electrical interface resistance was due to low inelastic surface scattering of electrons, and inelastic surface scattering was responsible for electromigration, and that Ge-overlayers on Cr-films also reduce the resistance.
In U.S. Pat. No. 3,801,325 Rasch and Cowden show that the resistance of a 2.5 nm thick silver film deposited on polyethylene terephthalate (PET) decreased from a resistance of 28 Ohm/square to a resistance of 25 Ohm/square if a 5 nm thick MgF2 overlayer was deposited on the Ag-film. This overlayer should not affect the resistance of the silver film because MgF2 is an insulator. Therefore one has again to assume that the interface between MgF2 and the silver film has a lower resistance than the surface of the silver film without an overlayer, and with it lower inelastic scattering. This reduction in resistance should lead to a reduction in the failure rate due to electromigration.